If you really want to pee in your diaper, some things to help are, of course drink lots of water. You can also try drinking something with caffeine in it, coffee is very good for having to make one pee, or tea. Caffeinated sodas will do as well. Alcoholic beverage will make you want to pee too, beer is always good.
I think a good thing to try is, when you feel you are ready, and you need to pee, close your eyes and imagine you are in the bathroom, or go stand at the bowl and imagine you are peeing normal. For me, being a guy, I would stand, close my eyes, and pretend I was peeing in a urinal. It helped me start the flow going.
After awhile you will get better with just letting go. And, have no fear, you won't unpottytrain yourself doing this. Or, you won't have ascendents because you wet some diapers!
One other thing, if you still find it hard to let go in your diaper. You can do a substitute diaper wetting. Just use some warm tap water, and pour it right into your diaper. You can do this while you are wearing them. Pull the diaper away in front, and pure till full! You can add some to the back as well. For that nice all around wet feeling. And doing this is cleaner too. No smelly pee, or chance for diaper rash. Hope this helps ya out? Have fun and enjoy!!!